The present invention relates to a base transceiver station provided with a transmitter unit, a receiver unit and a transmit path between the transmitter unit and an antenna, wherein the receiver unit determines output data from received signals and the transmitter unit converts input data into transmitted signals and pre-distorts the transmitted signals using at least one compensation value. Methods and software programs embodying the invention are also described.
Base transceiver stations for radio communication systems provide wireless connections, e.g. to user equipment. A radio communication system can be for example a mobile communication system, a satellite communication system or a radio system for point to point transmissions. The base stations comprise at least one transmitter unit which is connected over a transmit path to an antenna or to an antenna system with several antennas. The transmitter unit converts input data, i.e. information bits, into radio signals for transmission on a wireless link. The transmit path comprises generally at least one radio frequency filter and can include further components, e.g. an amplifier such as a tower mounted amplifier. A base transceiver station also comprises one or more receiver units and corresponding receive paths between an antenna or antenna system and the respective receiver unit. The receiver unit determines output data from received radio signals. In most cases, a receiver unit and a transmitter unit are combined in a transceiver.
The filters are often a part of a combining and distribution unit for the connection of one or more transmitter units or receiver units to one or more antennas. Parts of the receive and the transmit paths can be embodied by shared components, e.g. if the same antenna is used for transmitting and receiving signals or if a combining and distribution unit combines signals from different transmitters to an antenna and distributes received signals from an antenna to different receivers. Alternatively, the receive and the transmit paths can be separate if different antennas are used for transmission and reception.
For several modulation schemes, e.g. 8-PSK (phase shift keying) as used in EDGE (Enhanced Data rates for GSM Evolution) or quadrature amplitude modulation (QAM), a transmitted signal, also denoted as a symbol, comprises more than one information bit each. This can be achieved by mapping the symbols on a so-called I-Q plane, the points in the plane representing different phase shifts and amplitudes of transmitted signals. In these cases, the input data can be separated in the transmitter unit into an I-(inphase) and a Q-(quadrature phase) component. The components are again combined in the transmitter unit forming complex signals representing the information on the wireless connection.
The components in the transmit path distort the transmitted signals. Distortions which are acceptably small for a first transmission scheme, e.g. Gaussian minimum shift keying (GMSK) as used in GSM (Global System for Mobile Communication), may be too large for other modulation schemes. The distortions in the transmit path of a base transceiver station can therefore become a problem if the transmitter of the base transceiver station is updated for another transmission scheme, e.g. to allow an increased data rate.
A measure of the distortions is the error vector magnitude for which measurement procedures are defined for example in the specification Draft GSM 05.05, V8.3.0, European Telecommunications Standards Institute, 1999. Maximum values for the root mean square, the peak value and the 95th percentile of the error vector magnitude have to be observed by a base transceiver station according to this specification. For example, the maximum allowed root mean square error vector magnitude is 8% while the allowed peak error vector magnitude is 22%, both measured at the antenna reference point, i.e. the output of the transmit path to which the antenna is connected. In order to achieve high data rates, e.g. for EDGE, distortions should be as low as possible.
The distortion of transmitted signals can be reduced by a pre-distortion which takes components in the transmit path into account. For the correction of error vectors, a compensation value is used to calculate offsets for the transmitted signals. A compensation value can be determined for every type of component in a transmit path, for example for a combining and distribution unit which can be installed in a base transceiver station. When a component of said type is installed in the base transceiver station, the corresponding compensation value is entered into the transmitter for the pre-distortion of the signals transmitted.
Every individual of a given component distorts signals in a slightly different way and the distortion will especially vary for different revision states of the component. When a transmitter is upgraded and the base transceiver station has to observe stricter error vector requirements, correction values for already installed components may not be available while typical values for the respective type of component are not sufficiently exact. Furthermore, the performance of components can vary in time, e.g. due to aging or if tunable filters are used. These variations are not corrected by fixed compensation values. If more than one component is part of the transmit path, e.g. a combining and distribution unit with filters and a tower mounted amplifier, it is difficult to obtain a compensation value which considers the influence of all components.
Finally, distortions depend on the frequency of the transmitted signals. If a common compensation value is used for all frequencies of a transceiver, the correction can be insufficient for all or some of the frequencies. If, however, compensation parameters for all possible frequencies are stored, this requires a large memory which has to be updated if additional frequency bands are added.